Battery group size

[Aubbrey]

I recently purchased a used C-Dory 25' cruiser and I am needing to replace the batteries. My question is what group size is reccomended or is there that big of a difference between the 27 and 31. Also, i am going with the AGM versus the flooded cell if that has any impact on the group size. Any help is greatly appreciated.

[Robert H. Wilkinson]

What are your current batteries? Several variables. What are your house needs? What size engine? What cranking amps are required? How will they be recharged? Bigger is not always better if charging capacity is minimal. Colder climates also play a role requiring higher cranking amps.

I recommend a dedicated start battery and dedicated deep cycle for house.
Also marine rated has better support between plates for vibration.

Rob

[Tom Hruby]

Definitely go with the AGM for boat use. I have yet to find a "flooded" lead acid battery that does not leak a little in heavy seas. My first electric boat (29 years ago) had 12 flooded golf cart batteries, and I tried all kinds of tops/plugs. All of them ended up leaking to some degree, and the batteries needed to be topped off with distilled water regularly.

As already mentioned you should have a "starting" battery for your outboard and a "deep cycle" battery for your house load. The latter will last much longer when you need to discharge it below about 80% SOC (state of charge). Also note that batteries listed as both starting and deep cycle are a compromise and have all of the "negative" aspects of each type without being as good as each type separately.

This is what I found on Google [Group 31 batteries generally offer a higher cold cranking amp (CCA) rating compared to Group 27 batteries, meaning they are better at starting engines in cold weather. Group 31 batteries typically have a CCA range between 700-1150, while Group 27 batteries range from 600-1000]

[ssobol]

You need to be aware of where the batteries are mounted. Some boats have them in the lazarette which limits the size (due to the opening). My boat can only take size 24 because of this.

Yes, I could move the batteries if I wanted bigger ones, but the 24s work fine for me.

[gulfcoast john]

Congratulations on your new ride, great boat!

The ‘Group’ number is just an external case size dimension. Agree with Rob that the charger should also be adequate. My TC255 came with three Group 31 batteries, and a puny little Guest 10/5/5 charger.

If you anchor out a lot, you may want more battery power. If you find a deep-cycle battery that also meets ALL the specs for the engine (per your engine manual) you can ‘combine’ them for extra power on the hook. You don’t want to use a ‘starting’ battery for that, they are designed for massive amps out but only for way less than one minute. If you have the room, I’d suggest two Group 31 until you decide how you like to use your boat. The fridge compressor is the biggest overnight 12v hog on the boat, and many owners can get only one or two nights without recharging (with the engine in fast neutral or generator or solar panel). If you plan to have radar, rooftop AC, bicycles and kayaks up there you may not have room left for a solar panel.

So much depends on how you plan to use your boat…that makes it difficult to generalize too much. Certainly Lithium will become the House battery standard (few are rated as ‘start’ batteries yet), but they are not yet a ‘drop in’ replacement for your old batteries, and you may not want to pay an expert for that expertise after you just bought a CD-25.

Don’t fret…you’ll figure out what you NEED vs what you WANT just like the rest of us. PM or call if I can help, we’re on the Loop at the top of Michigan in SCA in the rain and I’m bored.

Safe Travels!
John

[thataway]

I have always switched to group 31 for the house bank. I do run with chest freezers/refers, so have that extra load. The group 31's fit between the 22's fuel tanks. On the last 25, I also had two 100 amp hour LiFePO4 batteries. These have amost twice the "power" of the group 27 (if not more power). You do have to have a DC To DC charger for the Li batteries.

If I was outfitting a 25 currently, I would go with the Li. There are some of the LiFePO4 which are actually cheaper than the AGMs now. Check Will Prowse's videos on the best Li batteries.

[colbysmith]

FWIW, on my 25, I run one group 24 or group 27 marine starting regular flooded wet cell battery. Then I have two group 31 AGM's in parallel for my house batteries. I was using two group 29 flooded wet cell batteries up until a month ago. The Fwc have worked fine for me in the past, but I think Walmart got a little cheaper in the make of their batteries and I was having an issue maintaining above 12 vts in them over night (12+hours). Also, I run a Cpap (without humidifier), refrigerator, portable freezer, webasto heater, and some fans and LED lights. The frig and freezer pull 3 amps each when they are running. The Webasto heater draws 8 amps for a short period of time when it is firing up or shutting down. I don't think my cpap draws more than an amp or two. And LED's are neglegible. AGM's cost about twice what flooded cell do. And the Lifepros I've heard about cost close to $1000. FLooded cell have been around for ever and unless you are running a lot of load, will probably do fine for the cost. The group size comes down to where your battery is stored and how big is the opening to get it there. For a house battery, I would go to the biggest size you can fit, realizing the bigger the battery, the more amphours and the longer it will take to recharge.

[thataway]

My experience is that most of the chest freezers and refrigeratos, draw is closer to 5 to 5.5 amps when running. The overall draw is possibly close to 3 amp over all. 24 hour run of either of these is about 60 amp hour--I have never seen 36 amp hours use which Colby suggests his get, I have been using these chest type units for almost 50 years not. First was in a Van Conversion Marie and I did in 1977. That Norcold chest lasted for over 20 years.

You can currently guy LiFePO4 100 amp batteries for about $250. I have four of them in my "household" back up system. They seem to be working well. Any good battery system needs monitoring. I have seen other brands for as low as $140 on Amazon currently.

Even if you use FLA or AGM, I advise the Victron 702, or for 1/4 the price a chinese copy (which I have also been using for several years and seems to work as well.). For an inverter, I would recommend Victron.

Renology 100 amp hour: $275, (I have been using one of these in my cars for everyday freezer work--bringing frozen food home from the grocery store)

SOK 100 amp hours batteries for $299.